Directional microphone



Jan. 30, 1951 H. F.'ol.soN

DIRECTIONAL MICROPHONE Filed Feb. 28, 1946 3 Sheets-Sheet l INVENToR. flaw? F /fon @l Jan. 30, 1951 H. F. OLSON DIRECTIONAL MICROPHONE 3 Sheets-Sheet 2 Filed Feb. 28, 1946 INVENTOR.

H. F. OLSON DIRECTIONAL MICROPHONE Jan. 30, 1951 3 Sheets-Sheet 3 Filed Feb. 28, 194e A 71777 mgm Patented Jan. 30, 1951 2,539,671 DIRECTIONAL MICROPHONE Harry F. Olson, Princeton, N. J., assigner to Radio Corporation of America, a corporation of Dela- Waffe Application February 28, 1946, Serial No. 650,979

9 Claims.

This invention relates to electro-acoustical alO- y paratus, and more particularly to a microphone the directional characteristics of which are symmetrical with respect to the microphone axis through 360 in a predetermined plane and the maximum pickup of which is confined to substantially that plane.

In certaintypes of sound pickup such, for examplejas in sound broadcasting or in picking up sound from an orchestra, the sources of sound are usually confined to substantially a single horizontal plane, the deviation from this plane being generally very small. Consequently, there is nothing to be gained by employing a non-directional microphone which picks up sound from all directions, including the sound reflected by the ceiling, the floor, etc. Now, the use of directional microphones in discriminating against noise and overcoming the effects of reverberation is well known. Many types of directional microphones have been proposed heretofore. For uniform directional pattern over a narrow frequency band, there is basically only one type, namely, that having'the limacon characteristic. This ranges from the bi-directonal cosine characteristic, through the cardioid characteristic and up to the nondirectional characteristic. Since, in certain uses such as those indicated above, the source of sound occurs in a plane over an angle of 360, the directivity indicated for this typeof pickup is a toroidal. directional pattern. t y

The primary object of my present linvention is to provide an improvedfmicrophone which will have a pattern of this sort.

More particularly, it is an object of lmy present invention to provide an improved directional microphone which is especially suitable for use Ainradiobroadcasting, sound reinforcing systems,v etc., wherel it'is desirable to have maximum pickup in substantially a predetermined plane but littl'e or no pickup outside of this plane.

Another-object ofv my present invention is to provide an improved microphone as aforesaid which has a directional. characteristic independent of the frequency of theA sound picked up thereby. ,t

A further` object of my present invention is to provide-an' improved microphone asV aforesaid" which will discriminate against random sounds,l such as'those resulting from reverberation and in planes at right angles to each other. Thus, the ribbons are arranged to vibrate in directions' at a right angle or normal to each other, the lines of maximum sensitivity of the ribbons lying alongv their said respective directions of vibration. The axis of the microphone is then along a line parallel to each of the ribbons and passing through the intersection of the aforesaid lines of maximum sensitivity. The outputs of the two ribbons or microphone units are connected in series electrically and a suitable phase shifting network is provided to bring the phase of the two ribbon outputs into quadrature 0r other suitable relation. When thus arranged with the outputs in quadrature relation, the directional pattern of the microphone assembly is a toroid having an axis com` mon With that of the microphone.

The novel features of my invention, both as to its organization and method of operation, as Well as additional objects and advantages thereof, will the ribbons of Figure 2 with the electrical con-l nection between them,v

Figure iv shows the directional characteristics of the individual microphone units ofr Figure 3,

as well as the directional characteristic of the combination or assembly of the two units, y

Figure 5 is a view similar to Fig. 2 showing the phase shifting networks in the output circuits of the two microphone units,

Figure 6 is a diagrammatic view, in plan, of the arrangement of Fig. 5,

Figure 7 shows the directional characteristics p of the individual microphone units of Fig. 6, as well as the directional characteristic of the combination of the two microphone.A units and their phase shifting networks in a plane` perpendicular to the microphone axis,

Figure 8 is a rear elevation of the two ribbons'KA of the arrangement of Figure 5 and showing the position of the two ribbons with respect to the microphone axis,

Figure 9 shows the directional characteristic of the microphone of Figure 8 in a plane containing the axis,`

Figures 10 and 11 correspond, respectively, to Figures 8 and 9, Figure 10 showing the ribbons of the two microphone units in perspective, and Figure 11 showing-the toroidal directional pattern of the microphone about the microphone axis as a center, and

corresponding parts throughout, there is shown,

in Fig. l, two microphone units A and B arranged side by side, but shown spaced apart somewhat for the sake of clearness. Each microphone unit includes a magnet I and a pair of associated pole pieces 3 having an air gap therebetween in which a conductive ribbon 5 is mounted for vibratory motion in well known manner. The microphone units are so arranged that the ux paths in the two gaps extend in planes normal to each other. The ribbons 5 are disposed in their respective gaps in the planes of the magnetic fields thereof, so that the ribbons are also disposed in planes normal to each other. The ribbons 5 are exposed to the atmosphere at both the front and rear surfaces thereof whereby they are responsive to the velocity or the pressure gradient component of sound waves which reach them from any external source.

It will be noted that both the air gaps and the ribbons 5 are aligned laterally with respect to each other so that they are disposed at the same elevation. The lines of maximum sensitivity of the ribbons 5 are represented by the lines 1, 'I of Fig. 2, the ribbons vibrating to and fro or back and forth along the direction` of these lines in response to the pressure gradient components of sound Waves impinging thereon. The axis of the microphone is represented by the line 9. As the ribbons 5 vibrate in their respective gaps, they generate signal voltage pulsations which correspond to the sound waves picked up by them. The electrical outputs of the two ribbons 5 are connected in series by the conductors IIA, I3 and I5.

As is well known, a velocity microphone has a bidirectional cosine characteristic. The Voltage output of such a microphone is given by the equation e=K cos (1) Where 0 is the angle between a line from the sound source to the microphone ribbon and the line of maximum sensitivity of the ribbon as a reference line, and K is a sensitivity constant.

If two velocity microphones A and B are placed at right angles to each other as shown in Figures 1, 2 and 3, the voltage output of the microphone A may be expressed by the equation and the voltage output of the microphone B may be expressed by the equation e2=K sin 0 (3) If'the ribbons 5 of two microphones are connected in series as shown in Figs. 2 and 3, the output of the combination may be expressed by the equation es=K (sin -i-cos 0) (4) Now, let

where ,b is a new reference axis. Substituting Equation 5 in Equation 4 l es=K [sin (=,!/-45)1c`os (gb-459)] (6) Simplifying Equation 6, it will be found that es=\/K cos 1,0 (7) Equation 7 shows that, in a plane perpendicular to the microphone axis, 9, the directional pattern of the two velocity microphone units A and B connected in series is still a cosine characteristic, but shifted 45 from the respective patterns or characteristics of the individual microphones A and B. This is shown in Fig. 4 where the characteristic of the microphone A is represented by the dot-and-dash line curve I9, the characteristic of the microphone B is represented by the dotted line curve I2 and the characteristic of the combi` nation in series is represented by the solid line curve I9.

Suppose, now, a phase shifting network Il' is connected in the outputs of each of the microphone units A and B, as shown in Figs. 5 and 6, to produce a quadrature relation or phase difference between the outputs of the two microphones. The ouput of the combination will then be given by the equation ef=K cos -l-jK sin 6 (8) The resulting characteristic of the combination in a plane perpendicular to the microphone axis 9 will then be as shown by the solid line curve I6 of Fig. 7. From this curve, it will be seen that a microphone including the phase shifting network iI will pick up sound with the same sensitivity in all directions in a plane perpendicular to the planes of the ribbons 5 (for example, in a horizontal plane when the ribbons 5 are disposed in vertical planes). However, the directional characteristic of this combination in any plane containing the axis 9 is a cosine characteristic, as shown by the curve I8 in Fig. 9. When viewed in perspective, the directional characteristic of the combination of the two discrete, Velocity microphone units A and B and their phase shifting networks I'I is a toroid the axis of which is coincident with the axis 9 of the microphone, as seen from Figure 1l. It is apparent, therefore, that my improved microphone will pick up sounds throughout 360 in substantially a planar region normal to the axis of the microphone and that it will discriminate against sounds originating at points external to this region.

In addition to discriminating against sounds coming from certain directions, my improved microphone will also discriminate against random sounds such, for example, as reverberation and general noise. In connection with directional microphones, the term directional efficiency is used to designate the ratio of energy response of such a microphone to the energy response of a non-directional microphone, all directions and phases of the incident sound being equally probable. The directional efficiency may be expressed as follows:

(91) Eff D..)=-,- 9)

9:0 (elim)2 where et is the voltage output of the directional microphone (see Equation 8 above), and eND is the voltage output of the non-directional microphone. In the case of my improved microphone, the directional efciency, or the ratio energy response thereof to that of a non-directional y microphone for sounds coming from random drections and of random phases is, from Equation 9, expressed as an integral as follows:

Where qs is the angle. between. thedirection4 ofV the:` incident sound. and .the plane normal to theaxis9, En is the energy response of my improved/ microphone for sounds originating in random directions, and` END is the energy response of the non-directional microphone for sounds originating in random directions.y

"Integrating Equation 1, it will be'vfound that" Nowsubstituting the limits to 1r/2 in Equation 11"; 'itwill be seen that.

El-2 ENIT/3 In other words, it is apparent from Equation 12 that'the energy response of". my improved microphone to `random sounds is-tWo-thi'rds that of a non-directional microphone;

'From' the foregoing description, it will be apparent tol those. skilled in the art that I have provided an improved microphone which is highly directional in substantially a plane and wi`l pick up sound in the region of said plane throughout 360. Although I have shown and described but a single embodiment of my present invention, it will undoubtedly be obvious to those skilled in the art that many variations thereof are possible. For example, the microphone units A and B may be so arranged that the ribbons 5, 5 will be disposed in planes related to each other by some angle which is either greater or less than 90 as may be found desirable. In such case, of course, the angle between the lines of maximum response 1, 'l will Vary correspondingly instead of being 90 apart, as shown in Fig. 5. Also,

Whether the ribbons 5 are disposed in planes normal to each other or not, they may be spaced more or less apart to suit particular requirements, although, in general, I prefer to arrange them side by side with their respective lines of maximum sensitivity 1, l at an angle of subn stantially 90 to one another. Moreover, the phase shifting networks I1 may be arranged so that they wll provide a phase difference in the output which is either greater than or less thanI 90, as may be found suitable. Where a phase diiference of approximately 90 is desired, a network such as shown in Fig. l2 may be employed, the constants of the inductors 2|, 25, 3l and 35 and of the capacitors 23, 21, 33 and 31 being suitably chosen. By varyingv these constants, a

phase difference other than 90 may be obtained.

As pointed out, this phase diierence may be varied over a wide range. phase may be varied from 45 to 135 with an output variation of only 3 db. Since many other modifications, as well as changes in those described, are possible, I desire to have it understood that the particular form of the invention herein described is to be taken as illustrative and not limiting.

I claim as my invention:

1. In electro-acoustical signal translating apparatus, the combination of a pair of vibratory membersl each responsive to the pressure gradient components of sound Waves and adapted to transform said waves into corresponding electrical pulsations, said members being laterally aligned and disposed for vibratory movement in directions angularly related to each other, means serially connecting the electrical outputs of said members, and means for establishing an out of For example, the

phase relation between the outputs of 'said memV bers.

2. In electro-acoustical signaltranslating apparatus; the combination of a pair of vibratoryA members each responsive to the pressure gradi`- ent'components of sound waves and adapted to transform said'. Waves into corresponding electrical" pulsations, saidv members being laterally aligned and disposed for vibratory movement in directions'normal to each other, means serialltrv connecting the electrical outputs of said members, and: means for establishing a quadrature beingf disposed for vibratory movement inA direc' tions normal to each other, means serially con.

necting the electrical outputs ofA said members, andl means for establishing a predetermined phase relation between the outputs of said mem' bers. Y

4. In electro-acoustical signal translating apparatus, the combination of means providing a pair of magnetic fields in planes angularly related to each other, a rst ribbon conductor mounted for vibratory movement in one of said fields. a second ribbon conductor mounted for vibratory movement in the other of said fields in a direction angularly related to the direction of movement of said first named conductor, said conductors being aligned laterally with respect to each other and being each responsive to the pressure gradient components of sound waves for transforming said waves into corresponding electrical pulsations, means serially connecting the electrical outputs of said conductors, and means for establishing a predetermined phase relation between the outputs of said conductors.

5. In electro-acoustical signal translating apparatus, the combination of means providing a pair of magnetic fields in planes angularly related to each other, a first ribbon conductor mounted for vibratory movement in one of said fields, a second ribbon conductor mounted for vibratory movement in the other of said fields in a direction angularly related to the direction of movement of said rstnamed conductor, said conductors being aligned laterally with respect to each other and being each responsive Vvto the pressure gradient components of sound waves for transforming said waves into corresponding electrical pulsations, a first output channel connected to said first conductor, a second output channel connected to said second conductor, a phase shifting network in each of said channels for shifting the phase of the output of its as` sociated conduct-or, and means connecting the outputs of said channels in series, said networks being adapted to provide a quadrature relation between the outputs of said channels.

6. Signal translating apparatus according to claim 4 characterized in that said magnetic fields are in planes normal to each other, and characterized further in that said ribbon conductors are arranged for vibratory movement in their re- Spective elds in directions also normal to each other. l

7. In electro-acoustical signal translating apparatus, the combination of a magnetic structure having a pair of laterally aligned, magnetic gaps arranged in planes at a right angle to each other, a' pair of ribbon conductors each in a separate one of said planes mounted for vibratory movement in their respective gaps also in directions at a right angle to each other, said conductors being also laterally aligned and being each responsive to the pressure gradient components of sound Waves for transforming said Waves into corresponding electrical pulsations, the respective lines of maximum sensitivity of said conductors being along their said respective directions, means serially connecting the electrical outputs of said conductors, and means for establishing a predetermined phase relation between the outputs of said conductors.

8. Signal translating apparatus according to claim 7 characterized in that said conductors are so disposed in their respective gaps that they are parallel to a common axis.

9. In electro-acoustical signal translating apparatus, the combination of a magnetic structure having a pair of magnetic gaps, a pair of ribbon conductors each mounted in a separate in, said conductors being arranged side by side in a manner such that (a) each is responsive to the pressure gradient components of sound Waves for transforming said waves into electrical pulsations, and (b) the respective lines of maximum sensitivity of said conductors are separated angularly, means serially connecting the electrical outputsl of said conductors, and means for establishing a predetermined phase relation between the outputs of said conductors.

HARRY F. OLSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,227,113 Campbell May 22, 1917 1,535,538 Maxeld Apr. 28, 1925 2,126,437 Williams Aug. 9, 1938 2,173,219 Anderson Sept. 19, 1939 2,230,104 Bostwick Jan.V 28, 1941 2,305,599 Bauer Dec. 22, 1942 2,408,395 Hays Oct. 1, 1946 

